Disease and trauma affecting the articular surfaces of the knee joint are commonly treated by surgically replacing the ends of the femur and tibia with prosthetic femoral and tibial implants, and, in some cases, replacing the patella with a patella component. Such surgeries are sometimes referred to as total knee arthroplasty (TKA). In TKA surgery, a surgeon typically affixes two prosthetic components to the patient's bone structure; a first to the patient's femur and a second to the patient's tibia. These components are typically known as the femoral component and the tibial component respectively.
The femoral component is placed on a patient's distal femur after appropriate resection of the femur. The femoral component is usually metallic, having a highly polished outer condylar articulating surface, which is commonly J-shaped. A common type of tibial component consists of a top surface (plateau) that generally conforms to the patient's resected proximal tibia. The bottom surface of the tibial component also usually includes a stem that extends at an angle to the plateau in order to extend into a surgically formed opening in the tibial intramedullary canal. Two common designs of the tibial component exist. In one design, the tibial component is monolithic (single piece) and made of plastic/polymeric material. In another design, a plastic or polymeric (often ultra high molecular weight polyethylene) tray is affixed on top of a tibial base plate which includes the stem and is usually made of metal. The top surface of the tibial component provides a surface against which the femoral component condylar portion articulates, i.e., moves in gross motion corresponding generally to the motion of the femur relative to the tibia.
While TKA is a highly successful surgical treatment option for severe knee joint disease such as osteoarthritis and rheumatoid arthritis, several studies have shown that current TKA implants do not restore the motion of the knee to the normal/healthy state, thus limiting patient function following surgery. Typically, the active (as opposed to passive when muscles are relaxed) range of knee flexion following TKA is limited to less than 115 degrees, whereas the healthy knee is capable of bending up to 160 degrees. Increased range of knee motion is required for activities like squatting and kneeling, which are particularly important for patients of certain ethnic and religious groups, as well as certain occupations and leisure activities. In addition to limited range of motion, complications, particularly of the patellofemoral joint, including chronic pain, patellar subluxation, patellar tilt, patellar dislocation and patellar component loosening have also been observed in 1-20% of TKA patients.
These limitations of TKA have in part been related to the inability of existing designs to replicate in vivo knee joint kinematics, including the kinematics of the femur relative to the tibia (tibiofemoral kinematics) and the patella relative to the femur (patellofemoral kinematics). The tibiofemoral kinematics following TKA are characterized by reduced posterior femoral translation and reduced internal tibial rotation, compared to normal knees. In addition, unexpected anterior femoral translation has been frequently noted in knees with TKA. Current TKA designs have also been shown to have abnormal patellofemoral kinematics. For example, studies have shown more superior patellofemoral contact, inconsistent patellar tracking, patellofemoral separation, and higher patellar tilt angles in TKA compared to normal knees.
Furthermore, many existing TKA designs only use kinematic information in 3 out of the 6 degrees-of-freedom, i.e., they include information regarding anteroposterior translation, internal-external rotation and flexion, but they do not include information about mediolateral translation, superoinferior translation and varus-valgus rotation. Information regarding superoinferior translation and varus-valgus rotation is particularly important to ensure that the tibiofemoral joint is not overstuffed in deep flexion, as often happens in patients receiving standard TKA. In addition, while TKA designs are beginning to incorporate kinematic features of the tibiofemoral joint, the patellofemoral joint has largely been ignored.
Accordingly, there remains a need for improved knee prostheses and methods for treating disease and trauma affecting the knee.